The invention concerns a method and means for maintaining and using a large capacity at a road network. It includes performing the method during time periods, when there are large traffic volumes and needs for large capacities. The method is focusing on reductions of blockings and risks for blocking of flows on links in a road network. One method step is limiting upstream flow to reduce risks for blocking of a downstrearn link. The method is using several method steps at different levels. Those steps cooperate to make a traffic management possible, which works in real time with traffic network functions.
Traffic volumes are large during rush hours, and there are built up queues at the road network in and outside large cities. It is difficult to get space for more roads and those are expensive to build. By using advanced information technology, the present capacity of the road network can be used more efficient and larger traffic volumes can be managed with less additions of new road capacity.
This matter is reflected by the large interest devoted for ITS, Intelligent Transport Systems. within EU, US and Japan and others during the nineteen nineties.
However there are no solutions known yet, why there are large amounts of money put into research in the field, and various ideas are studied.
Traditionally one has tried to solve capacity problems at the road network, by building more roads or taking actions at those points, where the problems appear. If there are long queues on a road upstream of an intersection, one is trying increasing the passability through the intersection for the cars on the said road. This is the traditional way of regarding traffic problems. The problems are narrow sections at the road network. At those points traffic queues are arisen, and then the solution is regarded to be increasing the capacity at those points.
With more knowledge in traffic and the network characteristics of traffic, the traditional xe2x80x9cpoint orientedxe2x80x9d way of work seems superficial and providing shortcomings. Resulting xe2x80x9csolutionsxe2x80x9d might create larger problems than the problem considered. An example is given as follows.
It is common that there are queues on the entrance roads of large cities during the morning rush hours. A queue might arise at a narrow section, eg at an on-ramp to the entrance road, and one might increase the passability here, e g by adding an extra lane. The resulting increased flow might come to a stop at a xe2x80x9cnewxe2x80x9d narrow section close downstream, whereby queues are created here instead. The queue at this new position might create larger traffic problems than the queue at the first position.
There are needs for a more system oriented way of work to solve traffic problems at road networks.
Methods described in literature are mainly concerning light signal controls of intersections. Most of the methods are point oriented and concern optimising one intersection for increased capacity. There are also various methods connecting controls of a few intersections along a larger traffic xe2x80x9carterialxe2x80x9d to produce a synchronised control of those intersections, a type of xe2x80x9cgreen wavexe2x80x9d. The green wave however is a xe2x80x9csolutionxe2x80x9d that is focusing on one of many aspects for a road network, like the corresponding xe2x80x9cpoint orientedxe2x80x9d way of work. The negative consequencies for traffic can grew large. It is also common to change methods from e g xe2x80x9cgreen wavexe2x80x9d to independent intersection control, depending on the traffic load In those cases a dilemma arises utilising the full intersection capacity. For timeplan controlled intersections there should be a full outflow at green signals from each inlink to the intersection. This can be done if each link contains queues, which supply the whole green periods with passing cars. Queues however are not desired from other points of views. They increase travel times (and drivers"" stomach acidity).
There are intersection controls, which to a larger extent adapt the green time length according to the amount of cars that are on the road. By measuring the flow a bit upstream, one knows if there are more cars arriving, and can increase the green time period correspondingly. In this way more green time can be taken from a link, that doesn""t need its share, to a link that needs more.
In short, there have been large efforts focused on obtaining increased passability through intersections. That is also a natural consequence of the matter, that a light signal controlled intersection only provides 20-40% of the inlink capacity. Variations in capacity depend on the intersection design, the share of left turnings, safety aspects and the used timeplan policy. The present invention also concerns providing large capacities. In the invention however the network orientation is dominating and system oriented solutions are invented. Those solutions consider the network capacity in a management coordinating way. Also in the present invention the capacity of a single intersection is of interest. But then it is related to other requirements and conditions.
The present invention differs already in applying a different problem view on traffic, compared with the traditional one, described above. The invention includes a new way of considering traffic problems, a new way of managing traffic and a new way of solving traffic problems.
We will start looking on some traffic problems, and we start simply with problems related to intersections, as such matters were discussed above.
Example on traffic problems.
Let us choose a light signal controlled intersection.
A link entering the intersection consists of two lanes, which closest to the intersection have been extended with an extra third lane, for those cars turning to the left. This extra lane has got space for five cars in a row. When the signal is green for cars heading straight, it is red for the left turning cars. When the left-turning lane is full of cars the rest of the left turning cars have to queue up in the ordinary left lane, why there only is left one lane for those heading straight. Then the passability is halved, and the cars that don""t get time to pass during the green period, are queueing up in both lanes.
When green for left turnings, those cars in the left-turning lane can pass the intersection. Those cars stopped behind in the queue, cannot pass the queueing cars in front of them and thus cannot utilize the green time for turning left. New cars are let in to the link from the upstream intersection. Those cars add on to the queue. When the light signal turns green again for going straight ahead, the passability once again is reduced, after that the left-tuming lane has been filled. Left-turning cars are blocking the xe2x80x9cstraight aheadxe2x80x9d flow, and xe2x80x9cstraight-aheadxe2x80x9d cars are blocking the left-turning flow. The capacity out from the link is decreasing, and if the capacity and flow in to the link is unchanged, the queue on the link is growing, until the queue is covering the whole link. Then cars from the upstream intersection cannot enter the link, although it is green light for the entrance roads to that intersection. This results in queues of stopped cars on the entrance roads, though the signal is green. Those queues in their turn block those cars on the entrance roads, that are heading for other roads out from the intersection. Thereby the outflows from all three entrance roads can be blocked, and their respective capacity turns to be very low, why the queues on those three links grow very fast and reach their respective upstream intersection, which in its turn is blocked, whereby its three entrance roads will be blocked and so on.
Observe that the blocking effect might give a very large gearing effect concerning the reduction of capacity. Say for instance that the first link (1) in the example above, gets its capacity reduced to ⅔. Upstream entrance road (2), which includes right-turing cars in to the first link, might get further reduced capacity. If right-turning cars consist of 20% of the flow of link (2), and ⅔ can pass into link (1), this at first gives a reduction of the total flow on link (2) of only 20% of ⅓={fraction (1/15)}. But the rest {fraction (14/15)} of the flow cannot pass the intersection freely, because right-turning cars are queueing and blocking one of the two lanes. If the rest, 80% of the original flow, is limited to one lane, the total capacity might be reduced by some further 30%, which might imply that the outflow from the link now is limited to some 20% of the basic link capacity (60% related to the node,xe2x80x94or less if also left-turing and straight-heading cars are blocking each other). That is valid when (2) has got two lanes. On smaller streets with only one lane, the blocking might reduce the capacity with 40%, i e the outflow from the link is small and queues can grow very fast upstream to the next intersection etc.
We see that blockings can cause large capacity reductions in a road network. Capacity reductions are also spread easily from a source on a link to whole areas of the road network. It is not required that there is an accident or a defective car, that causes the blocking. The natural cause at our large traffic volume city-areas, is traffic overloading. The inflow of traffic is larger than the link or the intersection can carry. See the following example.
We study the conditions in a four-road intersection. There is a timeplan for the traffic light control, which has been adapted to the normal traffic case and the normal distribution of traffic flows through the intersection. However the real traffic distribution is varying, and is statistically much different from the average distribution. This fact is remarkable during such short time periods as the green time periods of traffic lights, i e in the order of 30 seconds and below. It means that the integrated flow entering a link (1) from three entrance links (2, 3, 4) statistically might be remarkable larger than the average distribution. If this flow is let into (1), the in-flow is larger than the out-flow from (1), and a queue is built up on (1), with probable blocking consequencies as a result. By limiting the out-flows from (2, 3, 4) by the traffic lights, there are queues on those links instead, with possible blocking of traffic flows as a result. The blockings are decreasing the output capacity from the links, why the queues are extended and the blockings are maintained, and are spread across the network.
To get rid of a blocking, arisen on a link, the flow into the link has to be reduced below the level of the out-flow, and as the blocking has reduced the out-flow, there are substantial reductions required of the in-flow. That reduction of the in-flow to the link means reductions of out-flows for one or more upstream links. Thereby queue build up and blocking might arise on those links, whereby the in-flows to those links have to be reduced and so on.
The invention concerns a solution on the given problem above, blocking of traffic. One part is focused on reductions of in-flows in time, before blockings have arisen. Then there are less corrections required, and actions can be taken locally without larger consequencies for other parts of the network.
We see from the above example that there are not only incidents that are causing traffic problems. But also natural short term variations in traffic flows are sufficient to cause disturbances, which in their turn cause blocking effects. Those might grow large and be spread across the network. And one doesn""t need to prerequisite that something unusual must happen. It is sufficient with the large traffic flows on the roads into the city each morning, for queues to arise here and there. This is not by itself very serious. The large negative consequences are attained, when those queues create blockings. Then the queue growth rapidly increases and new blockings are created. Blockings are spread across the network. The result can be seen at the morning rush hours: When we need the large capacity of the road network the most, the traffic is blocked and the useful capacity is at its lowest level.
When a queue has grown upstream to a node and is blocking the node, it doesn""t help if the node is equipped with some of the above described light signal controls. It doesn""t help showing green light if traffic anyhow cannot move forward.
According to the view of the invention on problems, known traffic concepts and suggestions for solutions are mainly directed at the symptoms of the problems or problems of insignificance. Most of the advanced work done, to fine tune the capacity in light signal controlled intersections, turn unuseful in real overload situations and blockings.
Simply expressed. the traditional methods are based on working downstream with the traffic. E g successively removing narrow sections, whereby the flow is increased downstream, until it is caught by a new narrow passage. This strategy doesn""t reach success, until the whole network is expanded to such a large road capacity everywhere, that traffic, not even when reaching its worst peaks, reaches the capacity limit anywhere. This case is and has been unrealistic to attain for large cities the last decades.
Traffic planners"" usual attitude is xe2x80x9cgiving inxe2x80x9d saying, that they don""t get rid of the queues, because increasing the passability, the traffic increases, and there will be queues anyhow.
According to the invention, the successful strategy is working with management systems in control of traffic, and then working opposite to the traditional way. A main principle is wordking upstream against the direction of the traffic.
Simply stated, the system shall not let through more traffic into a link or through a node than the following link or node can handle. This means that the control requirements are transferred upstream. The out-flow from a link may need to be limited, as a downstream link doesn""t cope with the whole in-flow. But if this link out-flow is made limited, also this link in-flow has to be limited correspondingly, and thereby also upstream links might need to be limited. This upstream feedback of limitations of flows is necessary, to prevent creations of blockings. Blockings also are spreading upstream. So the traffic control has to be faster, and be able to be fed upstream faster than the blockings are spread.
The fundamental principle for traffic control is; don""t let more traffic in than what can pass out. This means consequently, that it is interesting to try to increase the xe2x80x9coutput trafficxe2x80x9d. In such a case more traffic can be put in. The question if more traffic can be put in, however is not that simple just looking at the exit point, which is the traditional way, and as said above might lead to advanced optimisations of e g an intersection control to increase the output from a link.
No, first one has to study the road network downstream. Will that be able to handle the increased inflow, which will be the result of the said increased output flow? If the answer is no, the result of the fine tuning of the said intersection control mightxe2x80x94(not be meaningless), but negative. If it would be carried out, a queue would be growing up to the intersection and block that one. Thereby the intersection obtain still less capacity than what it would have initially, and instead of getting increased output flow, it gets decreased flow. The result thus becomes opposite to what was the purpose.
According to the fundamental principle, the management system shall instead rapidly make upstream feed back of the decreased output, and limit the in-flow to the link by limiting the out-flow of upstream links.
And the management system has to react fast, before queues and blockings have spread further upstream.
It is first when upstream flows have decreased that much, that the queues have begun to decrease and the original blocking has resolved, that the traffic flows can be increased again to the original level.
A prerequisite for increasing the flow at a point is that downstream parts of the road network can handle the extra flow. This is completely according to the fundamental principle and implies that the control requirements are applied in the upstream direction.
The purpose: The invention makes possible the solution of the large traffic problems, which characterise the traffic in the large city areas of today. The invention identifies the major problem and provides a method and means for solution of the major problem.
The invention concerns a method and means to maintain and utilize a large capacity in a road network. It includes performing the method during time periods when the traffic volume and the needs for capacity are large. The method is concentrated on reduction of blockings and risks for blockings of flows on links in a road network. A method step is to limit upstream flows to reduce risks for blocking of downstream links. The method is using several method steps at different levels. Those steps cooperate to make traffic management possible, that works in real time with the network characteristic functions of traffic.